one durable object per room or one DO plus a KV index for 500 live rooms Durable Objects

One per room, without hesitation. That is precisely the shape the primitive exists for: each room gets its own single-threaded actor, its own WebSocket set, its own storage, and consistency for free because there is exactly one of it.

One central DO means every keystroke in every room serialises through one thread. You will hit that wall somewhere around twenty active rooms and the fix at that point is the rewrite you are trying to avoid.

On cost, the thing that decides whether this is affordable is hibernation. Use state.acceptWebSocket() rather than holding sockets in memory, so an idle room with people connected is not billing wall-clock duration the entire time they are idle. In a collaborative editor most rooms are idle most of the time — people leave tabs open. Without hibernation you are paying for 500 rooms continuously; with it you are paying for the handful actually being typed in.

Listing rooms is a separate problem and should not influence this. Keep room metadata in D1 and let the DOs handle only live state.

Second the per-room design, and be careful where you put the index. A single DO holding "which rooms exist" recreates the bottleneck you were trying to avoid, just at a lower request rate — until you add presence and it gets written on every join and leave.

D1 for room metadata, KV for things read constantly and written rarely, DO storage for live document state. Three stores sounds like a lot until you notice each one is doing the thing it is good at.

Numbers from something similar, since "is it expensive" deserves a real answer: about 300 rooms, hibernating sockets, storage in the low hundreds of megabytes total. Bill sat under ten dollars a month and roughly 80% of it was request count, not duration. The cost model punishes chattiness far more than it punishes having many objects.

Think about placement before you have users in two hemispheres. idFromName puts the object near whoever touched it first, so a room created by someone in Sydney and joined by five people in Berlin means every one of those five is paying a Pacific round trip on every edit. If rooms are regional in nature, look at location hints at creation time.

Make sure you can clean up. Objects are cheap, storage you never delete is a slow leak, and there is no button anywhere that says "drop everything". Decide the deletion story on day one while you still only have test rooms.